Fuel injector assemblies are useful for such applications as a gas turbine combustion engines for directing pressurized fuel from a manifold to one or more combustion chambers. Such assemblies also function to prepare the fuel for mixing with air prior to combustion. Each such injector assembly has an inlet fitting connected to a manifold, a tubular extension or stem connected at one end to a fitting in a typically manifold, a tubular extension or stem connected at one end to a fitting in a typically cantilevered fashion, and one or more spray nozzles or nozzle tips connected to the other end of the stem or housing for directing the fuel into the combustion chamber. A single or multiple fuel feed circuits extend through the housing to supply fuel from the inlet fitting to the nozzle or nozzle assembly.
In a typical fuel nozzle, active fuel flow passages are surrounded by an insulation cavity, or tertiary, to prevent excessive heat transfer. Such a fuel nozzle insulation cavity is only partially sealed since a completely sealed fuel cavity, if there is any leakage of fuel from the associated braze joints or O-ring seals, upon absorbing surrounding heat, can cause structural breakdowns or even a possible explosion. Typically, vents are provided to prevent such undesired high pressure buildups, generally via an annular gap between the nozzle shroud and the fuel body which functions to transfer the leaked fuel and/or fuel/air mixture downstream of the combustion liner. However, due to the nature of the transient aerodynamics, air pressure in the tertiary can be lower than the pressure at the vent exit and, as a result, there can be undesired backflow of the combustion products, fuel and fuel/air mixture from the combustor into the tertiary. In order to prevent such backflow, a positive air wash is typically used and backflow is usually prevented. However it cannot be guaranteed that the tertiary pressure is always lower than that of the compressor discharged air and thus it is possible that the noted leaked fluids can actually escape from the air wash inlet holes rather than flowing to the vent exit. The present invention addresses this unsolved problem.
An attempted prior art solution is set forth in U.S. Pat. No. 5,615,555 to Mina, which discloses a fuel injector for a gas turbine with means to prevent flashback. The utilized injector includes an outer shroud that forms an annular passage for directing compressed air towards the downstream end thereof and to additionally direct fuel spillage from purging holes toward the downstream end thereof. However, this proposed solution more closely resembles the existing state of the art and thus retains the noted described weaknesses. Specifically, gas fuel in chamber 1 can flow upstream through an aperture 17 in some transient situations. The present invention minimizes this potential via a pneumatic resistance of an air transfer sleeve.
While U.S. Pat. No. 4,198,815 to Bobo et al. pertains to air assisted fuel atomization, it is otherwise not relevant to the present invention.